1. Field of the Invention
This invention relates to the separation of carbon dioxide and other aggressive gases from gas mixtures at low temperatures.
2. Prior Art
It is known to separate one gas from a mixture of gases by using a membrane more permeable to that one gas than the other gases of the mixture. The gas mixture is brought into contact with one side of the membrane and, under a pressure differential across the membrane, the one gas permeates to the other side of the membrane and is withdrawn. Frequently, these membranes are in the form of hollow fibers.
Often gas mixtures for which a membrane gas separation is desired may contain one or more species which may, under certain conditions, be detrimental to the operating characteristics of the membrane. Such species may adversely effect the membrane by solvation, plasticization, swelling, etc. The adverse effect may be a significant change in the permeability of the membrane for one or more of the gases of the mixture. In some cases, the permeability of the membrane changes for all gases in the mixture in such a manner that all of the gases pass rapidly through the membrane without any separation occurring. These adverse effects may be rapidly catastrophic or may be manifested as a gradual degradation of desired membrane properties over time. These adverse effects can usually be monitored by measuring permeability creep, i.e., the change in permeability, over a period of time. Depending on the particular membrane, adversely effecting species, such as ammonia, may be detrimental to the membrane even if the concentration, measured in terms of partial pressure, of the adversely effecting species is low. In other cases, such as the separation of an acid gas such as carbon dioxide or hydrogen sulfide from hydrocarbon gases such as methane, the acid gas is often present in substantial concentrations and is the predominant permeating species. In this regard, it is known that an acid gas, such as carbon dioxide, can under some conditions plasticize an otherwise rigid structure of a polymeric material such as polysulfone.